This invention relates generally to electrical circuits, and more particularly, the invention relates to a current mirror circuit with temperature compensation.
The current mirror is often used in bipolar circuits such as low voltage bipolar amplifier circuits. A problem in low voltage applications stems from variations in base-emitter voltage, Vbe, with temperature, which can adversely affect a reference current and a mirrored current. In many applications, including power amplifiers, current mirror circuitry is used to set up an operating current through the amplifier. With a low operating voltage using a three cell nickel battery or lithium ion battery, it becomes difficult to maintain a constant current over a temperature range. Unfortunately, changing the operating current over temperature will adversely affect circuit performance.
For example, FIG. 1 shows a prior art current mirror circuit used to set the operating current for a rf power amplifier transistor, Qrf, A reference current, Iref, flowing from a reference voltage, Vref, to circuit ground through resistor Ra and transistor Q2 is mirrored (proportionately to transistor sizes) as current Icrf through rf power transistor Qrf -between voltage Vcc and ground. Transistors Q3, Q4 connect a bias voltage, Vbias, to the respective base regions of transistors Q2, Qrf due to the same bias voltage, VA, being applied to transistors Q3, Q4. As temperature rises, the current through Qrf increases as Vbe is reduced at about xe2x88x922 mV/xc2x0 C. for a silicon bipolar transistor or xe2x88x921 mV/xc2x0 C. for a gallium arsenide heterojunction bipolar transistor, GaAs HBT. When this circuit is used in a CDMA handset power amplifier, the increased bias current of Qrf at high temperature is unfavorable for xe2x80x9ctalk timexe2x80x9d.
The present invention is directed to compensating for temperature induced variations in a current mirror circuit, such as shown in FIG. 1, for example.
In accordance with the invention, in a current mirror circuit having a reference node and a reference current transistor connected to the reference node, a temperature compensating circuit for maintaining the reference current value during temperature variations includes a compensating transistor connected between the reference node and circuit ground and parallel with the reference current transistor. Bias circuitry is provided for biasing the compensating transistor whereby current flows from the reference node to ground through the compensating transistor in order to remove excess current when temperature increases.
In a low voltage bipolar transistor amplifier circuit embodiment, a power bipolar transistor is connected between a supply voltage and circuit ground, and a current mirror circuit including a reference current transistor is serially connected with a first resistor between a reference voltage and circuit ground. A pair of bipolar bias voltage transistors are connected respectively between a bias voltage and a base of the reference current transistor and between the bias voltage and a base of the power bipolar transistor, the bases of the pair of bipolar bias voltage transistors being connected to the common terminal of the reference current transistor and the first resistor. The temperature compensating circuit including the compensating transistor is then connected between the reference node and circuit ground in parallel with the reference current transistor with bias circuitry biasing the compensating transistor whereby current flows from the reference node to ground through the compensating transistor to remove excess current from the node when temperature increases.
In another embodiment of the invention, the bias circuitry for the compensating transistor includes a diode for limiting bias circuitry current flow when the reference voltage drops below the base-emitter voltage of the compensating transistor.
In another embodiment, a switch transistor can be connected in parallel with the reference current transistor to reduce the reference current transistor when the switch transistor is conducting.
The invention and objects and features thereof will be more readily apparent from the following detailed description and dependent claims when taken with the drawings.